This invention relates to a method for electrically agitating a molten metal and to a furnace for providing effective agitation of a molten metal.
For the sake of convenience, the invention will be described hereinafter with reference to molten aluminum as a representative of all possible molten metals with which the present invention can be used.
Generally for the melting of metallic aluminum, there is used a reverberatory furnace the operating principle of which resides in heating the furnace ceiling and side walls such as by means of a burner and utilizing the radiant heat from the ceiling and side walls for melting the metal held inside the furnace. In this case, if the metal fused by the heating (hereinafter referred to as "molten metal") is held stationarily inside the furnace, the heat generated for the melting is transferred solely by conduction, with the result that the heat readily reaches and superheats the outer region of the entire body of molten metal but reaches the inner region thereof only after a long time. When the heat is thus conducted, it will be very long before the entire body of the metal is completely melted to the core. To expedite the melting of the metal, therefore, it becomes necessary for the molten metal to be amply agitated inside the furnace.
Agitation given to the molten metal results in a notable reduction in the time required for melting the metal, because the agitation not only eliminates local differences of temperature in the molten metal but also enables the heat to be transferred by convection. In the melting of aluminum scraps, for example, the reduction in the time required for heating decreases the duration of the exposure of the metal to the combustion gas of an elevated temperature and curtails possible melting loss of the metal.
As one way of agitating a molten metal, there has heretofore been adopted a manual means of agitation such as by use of a suitable implement like a metal bar.
This method has a disadvantage as indicated below.
Since the furnace charged with the molten metal is kept at highly elevated temperatures, the operation is required to endure bad working conditions and the manual work performed by the operator does not provide thorough agitation for the entire volume of the molten metal.
Further, the iron and the like of which the metal bar is made cannot be prevented from being melted into the molten metal and consequently impairing the composition of the molten metal. Particularly in the manufacture of an aluminum alloy, no such inclusion into the alloy of the iron can be tolerated.
There has been adopted another method which comprises forcefully blowing an inert gas such as argon or nitrogen into the molten metal so much as to cause agitation of the body of molten metal. When molten aluminum is agitated by forced blowing of nitrogen, there ensues formation of aluminum nitride which eventually collects in the form of scum on the surface of the molten aluminum. Some of the aluminum nitride is trapped within the body of molten aluminum and inevitably degrades the composition of the molten aluminum.
From the practical point of view, it is extremely difficult for the molten aluminum to be thoroughly agitated solely by the forced blowing of such an inert gas.
An object of the present invention is to provide a method for the agitation of a molten metal without entailing the least change in the composition of the molten metal.
Another object of the present invention is to provide a method for the agitation of a molten metal without exposing the operator to a bad working environment.
Still another object of this invention is to provide a device for effective agitation of a molten metal without entailing the least change in the composition of the molten metal.
A further object of this invention is to provide a device for effective agitation of a molten metal without exposing the operator to a bad working environment.